1. Field of the Invention
The present invention relates to a method of rejuvenating chlor-alkali cells, and more particularly it relates to rejuvenating the cathodes of chlor-alkali cells having a membrane separating the cell into distinct anolyte and catholyte compartments.
2. Description of the Prior Art
The production of chlorine and caustic from brine by electrolysis is well known and is an important commercial process. In general, the process was either carried out with a group of mercury cells, or with the use of permeable diaphragms such as asbestos diaphragms. A recent development is the use of an ion exchange membrane in place of the permeable diaphragms. It is to the latest type of cells having membrane separators that the present invention pertains.
In the prior art diaphragm cells, one solution, generally saturated sodium chloride solution, is flowed first into the anode compartment and then through the porous diaphragm into the cathode compartment of the cell. This operation requires only one solution feed, and therefore a purification of only one solution is necessary. By purification, it is meant the removal of calcium and magnesium ions in the feed solution, and this purification was generally considered necessary for efficient cell operation. In the new technology which utilizes a membrane as a separator between the anode compartment and the cathode compartment, a separate anolyte solution and a catholyte solution of different chemical makeup is required. In other words, sodium chloride is still fed into the anode compartment, but water is generally fed into the cathode compartment.
It is well known that calcium and magnesium ions can form insoluble materials in diaphragms and membranes and therefore such ions in the anolte solution now are a source of a rather serious problem in this type of electrolytic cell. However, calcium and magnesium ions in the cathode compartment have not been considered serious in this respect because they tend to migrate toward the cathode rather than toward the membrane.
Accordingly, the prior art contains considerable technology for rejuvenating diaphragms and for handling the plugging problem caused by calcium and magnesium ions. For example, U.S. Pat. No. 1,309,214 mentions withdrawing electrolyte, and washing the cell with acid such as lactic acid so as to remove the calcium and magnesium from the cell. U.S. Pat. No. 3,467,586 teaches a method for rejuvenating asbestos diaphragms by washing with strong acid until the solution passing through the diaphragm is below ph 8. The procedure calls for draining the cathode compartment, and although acid reaches the cathode chamber, it is not clear whether the acid completely washes the cathode surface or not. These processes also have the disadvantage of requiring cell shut down during the rejuvenating procedure. U.S. Pat. No. 3,220,946, which shows a three compartment cell, mentions adding acid to the middle or buffer compartment to prevent back migration of hydroxyl ion. However, the catholyte is not acid and acid doesn't reach the cathode.
These techniques have also been utilized to rejuvenate the membranes in membrane cells, and U.S. Pat. No. 4,040,919 calls for the addition of hydrochloric acid to the anolyte and water to the buffer or catholyte to decrease the pH in the cathode compartment. However, the pH of the cathode compartment is still strongly alkaline. U.S. Pat. No. 4,115,218 discloses a method of electrolyzing brine wherein the membrane is purified by periodically reducing the electric current and flooding the cathode compartment. However, this process requires that the water utilized be rather highly purified by substantially complete removal of calcium and magnesium ions.